1. Field of the Invention
The present invention relates to a drying apparatus and, more particularly, to a drying apparatus for an organic solvent that is used for manufacturing a photosensitive material or a magnetic recording medium.
2. Description of the Related Art
Generally, photosensitive materials and magnetic recording media are manufactured through a drying process after a predetermined coating liquid such as a magnetic liquid is applied on a continuously traveling support (hereinafter referred to as a web), so that an organic solvent in the coating liquid is evaporated by a drying apparatus to dry the coating liquid.
Such a drying apparatus for an organic solvent requires that an organic solvent concentration (hereinafter referred to as a gas concentration) of an exhaust gas containing the organic solvent is kept sufficiently lower than a lower explosive level (LEL). If the gas concentration cannot be kept lower than the lower explosive level, expensive explosion proof equipment must be provided to secure safety. Especially, in view of explosion proofing safety of the drying apparatus, a blower is a device to be an ignition source, and it is important to keep a low concentration of a gas introduced into the blower.
The drying process is generally divided into three periods: a remaining heat period, a constant rate period, and a falling rate period. In drying an organic solvent that has a relatively low boiling point and is used for a magnetic recording medium, the solvent often dries quickly, and the organic solvent rapidly evaporates from a coating liquid. Specifically, the constant rate period is extremely short to soon reach a later stage of the falling rate period. To avoid this danger, it is necessary to increase the amount of fresh air introduced into the blower, and also necessary to balance the amount of exhaust air. In this case, even if only a part of the drying apparatus is in danger in view of explosion proofing, it is necessary to balance the amount of exhaust air in the entire drying apparatus. This reduces efficiency of the drying apparatus to require a complex device that can set the amount of supplied and exhausted air for each drying zone, thus increasing the weight and the length of the drying apparatus.
Japanese Utility Model Publication No. 6-28221 discloses a gas concentration control that controls a gas concentration of a drying apparatus for an organic solvent to be kept lower than a lower explosive level includes an inner pressure/solvent gas concentration control device in a dryer.
This dryer includes a plurality of drying zones, and ducts of a supply system and an exhaust system connect to each drying zone. In the supply system, a circulation air from each drying zone by circulation piping and a fresh air by supply piping are mixed at a constant ratio to supply the mixed air to the drying zone by a circulation blower. In the exhaust system, an exhaust blower with a variable speed control motor provided in the exhaust duct joins an exhaust gas to exhaust piping. The joined gas is exhausted by a main exhaust blower, and damper opening control by a damper with an opening control motor provided in the exhaust system is automatically adjusted based on measurements of a gas concentration meter placed midway through the exhaust duct, thus a solvent gas concentration of the exhaust gas is controlled to be lower than the lower explosive level.
However, in the conventional drying apparatus in Japanese Utility Model Publication No. 6-28221, when the organic solvent rapidly evaporates in one of the plurality of drying zones, an exhaust gas with an extremely high gas concentration is circulated to the circulation blower, which may cause a dangerous operating condition in view of explosion proofing without a large amount of fresh air mixed by a main supply blower. As a result, a main supply blower and a circulation blower having a large capacity must be provided to increase equipment costs and running costs. The method in which the damper opening control by the damper with the opening control motor is automatically adjusted based on the measurements of the gas concentration meter placed midway through the exhaust duct to keep the gas concentration of the exhaust gas to be lower than the lower explosive level increases the size of the explosion proof system, apparatus costs and running costs.
The present invention has been achieved in view of the above and has an object to provide a drying apparatus that has a simple apparatus configuration and reduces a concentration of a gas introduced into a circulation blower and an exhaust blower to simplify an explosion proof system and significantly reduce equipment costs and running costs.
To achieve the above-mentioned object, the present invention is directed to a drying apparatus, comprising: a plurality of drying chambers arranged in line through which a web with a coating film applied thereon travels to dry the coating film; a supply system having in each drying chamber a circulation blower which supplies a dry air that is a mixture of a fresh air and part of exhaust gases circulated from the drying chambers; and an exhaust system which exhausts the exhaust gas from each of the plurality of drying chambers, wherein the exhaust system comprises: a joining duct to which the exhaust gases from the plurality of drying chambers are joined; an exhaust blower provided in the joining duct; and a plurality of exhaust ducts provided in each drying chamber and having inlets at different positions in the drying chamber, wherein the plurality of exhaust ducts form an independent exhaust passage comprising a duct for circulating the exhaust gas to the circulation blower, and a duct for directly feeding the exhaust gas to the joining duct.
According to the present invention, in the exhaust system, the exhaust gases from the plurality of drying chambers are joined to the joining duct to be then exhausted by the exhaust blower, thus the gases having high or low concentrations depending on the drying chambers are joined to the joining duct to dilute the exhaust gas from the drying chamber having the high gas concentration with the exhaust gas from the drying chamber having the low gas concentration. The diluted exhaust gas is introduced into the exhaust blower, thereby allowing a gas concentration in the exhaust blower to be an ignition source in view of explosion proofing safety to be kept lower than a lower explosive level with a simple apparatus configuration.
The plurality of exhaust ducts having the inlets at the different positions in the drying chamber are provided to form the independent exhaust passage comprising the duct for circulating the exhaust gas to the circulation blower, and the duct for directly feeding the exhaust gas to the joining duct. This configuration allows the exhaust duct having the inlet in the lowest gas concentration area to be used as a duct for the exhaust gas circulated to the circulation blower, and allows the exhaust duct having the inlet in a high gas concentration area to be used as a duct for the exhaust gas directly fed to the joining duct. This prevents the exhaust gas with the high gas concentration from being introduced into the circulation blower. Therefore, the gas concentration in the circulation blower to be an ignition source in view of explosion proofing safety can be kept lower than the lower explosive level with a simple apparatus configuration.
It is preferable to provide a switching device for optionally switching communication targets of the plurality of exhaust ducts between the circulation blower and the joining duct. In this case, it is more preferable to provide a gas concentration meter in the plurality of exhaust ducts so that the exhaust duct having the inlet in the lowest gas concentration area communicates with the circulation blower based on measurement results.